1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to a color image forming apparatus employing an intermediate transfer method.
2. Discussion of the Background
Recently, demand for color image forming apparatuses, such as color copiers and color printers, has been increasing in the image forming apparatus market.
An electronographic color image forming apparatus can employ a tandem method. A tandem image forming apparatus includes a plurality of image carriers (e.g. photoreceptors) that are accompanied by an image developer of each color. On each of the image carriers, an electrostatic latent image of single color is formed and developed into a toner image with toner.
In case of an intermediate transfer method, the image forming apparatus further includes an intermediate transfer belt, a primary transferer, and a secondary transferer. The single color images can be transferred and superimposed on the intermediate transfer belt one on another by the primary transferer with Coulomb's force due to a primary transfer electric field to form a synthesized color image. The color image (toner image) is then transferred onto the recording medium by the secondary transferer with Coulomb's force due to a secondary transfer electric field.
The primary and secondary transfer electric fields are formed to desirably act in a transfer nip where the photoreceptor and the intermediate transfer belt or the intermediate transfer belt and the recording medium are in close contact with only toner also being present. Otherwise, a discharge phenomenon is likely to occur, which can deteriorate image quality.
For example, if the electric field acts upstream of the transfer nip, the toner on the intermediate transfer belt can scatter onto the intermediate transfer belt or the second medium (pre-transfer scattering). Further, the toner previously transferred on the intermediate transfer belt can be transferred onto the image carrier during transfer of a second toner image and subsequent toner images (reverse transfer).
Fluctuation of a charge amount of toner can cause the toner scattering and the reverse transfer. The fluctuation can be caused by ion migration due to discharge and charge transfer caused by electrostatic induction in or near the primary transfer nip.
The above problems can be reduced by decreasing the transfer bias voltage. However, the toner transfer rate may decrease and the amount of toner that fails to be transferred (remaining toner) may increase when the transfer bias voltage is reduced and becomes insufficient. Because of these problems, a need exists to limit the electrostatic induction phenomenon and to reduce discharge to desirable levels so as to obtain desirable image quality.
To achieve the above, an exemplary image forming apparatus can include an image carrier, an intermediate transfer belt, a primary transferer, a secondary transferer, a first contact member, and a potential maintaining member. After passing by the primary transferer, the intermediate transfer belt next contacts the first contact member whose surface potential can be maintained not less than the charge potential of a back surface of the intermediate transfer belt by the potential maintaining member. Alternatively, the image forming apparatus can include a bias applicator to maintain the level of surface potentials of all components that the intermediate transfer belt contacts after the primary transferer up to the secondary transferer. The level is maintained to be not less than the charge potential of the back surface of the intermediate transfer belt.
Another exemplary image forming apparatus can include an image carrier, an intermediate transfer belt, an electrostatic transfer member, a pre-transfer prevention member, and a remaining toner transfer prevention member. The electrostatic transfer member, the pre-transfer member, and the remaining toner transfer prevention member can be provided at an opposite side of the image carrier with respect to the intermediate transfer belt. The electrostatic transfer member can generate an electric field to transfer a toner image from the image carrier onto the intermediate transfer belt.
The pre-transfer prevention member can be provided upstream of the electrostatic transfer member and the remaining toner transfer prevention member can be provided downstream of the electrostatic transfer member in a rotation direction of the intermediate transfer belt. A bias voltage of the same polarity as the polarity of the toner can be applied to the pre-transfer prevention member and the remaining toner transfer prevention member.